Hybrid photonics-solid state quantum computer

ABSTRACT

There is described herein a quantum computing system comprising a quantum control system configured for generating microwave signals up-converted to optical frequencies, at least one optical fiber coupled to the quantum control system for carrying the up-converted microwave signals, and a quantum processor disposed inside a cryogenics apparatus and coupled to the at least one optical fiber for receipt of the up-converted microwave signals. The quantum processor comprises at least one optical-to-microwave converter configured for down-converting the up-converted microwave signals, and a plurality of solid-state quantum circuit elements coupled to the at least one optical-to-microwave converter and addressable by respective ones of the down-converted microwave signals.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 17/499,925 filed on Oct. 13, 2021, which claims the benefit ofU.S. Provisional Patent Application No. 63/124,761 filed on Dec. 12,2020 and U.S. Provisional Patent Application No. 63/225,963 filed onJul. 27, 2021, the contents of which are hereby incorporated byreference.

TECHNICAL FIELD

The present disclosure generally relates to the field of quantumcomputing. More specifically, the present disclosure relates to controland readout of solid-state qubits such as superconducting qubits, spinqubits, and topological qubits.

BACKGROUND OF THE ART

Solid-state qubits such as superconducting circuits, spin qubits andtopological qubits based on semiconductor/superconductor nanowires areamong leading architectures to build a quantum computer. Quantum controland readout of such qubits typically involve use of electronics andwaves in microwave frequency (GHz) regimes.

To protect the qubits against thermal noise, these qubits are placed incryogenics systems and operated in ultra-low temperatures, typically ofthe order of few millikelvins above absolute zero.

The qubits are controlled and measured by generating microwave pulses atroom temperature and delivering the waves to the qubits in the cryostatvia a set of microwave coaxial cables. When measuring the qubits, theinput microwave pulses interact with the qubit circuit to produce anoutput microwave signal which is then transmitted to readout electronicsby another coaxial cable. The coaxial cables are thermally anchored todifferent cooling stages of the cryogenics system. A quantum processorusually requires at least one control coaxial line per qubit forperforming single-qubit gates on top of a number of readout coaxiallines. Additional coaxial lines can also be required to control couplersused to implement multi-qubit gates.

While existing techniques for controlling and measuring qubits aresuitable for their purposes, improvements are desired.

SUMMARY

In accordance with a first broad aspect, there is provided a quantumcomputing system comprising a quantum control system configured forgenerating microwave signals up-converted to optical frequencies, atleast one optical fiber coupled to the quantum control system forcarrying the up-converted microwave signals, and a quantum processordisposed inside a cryogenics apparatus and coupled to the at least oneoptical fiber for receipt of the up-converted microwave signals. Thequantum processor comprises at least one optical-to-microwave converterconfigured for down-converting the up-converted microwave signals, and aplurality of solid-state quantum circuit elements coupled to the atleast one optical-to-microwave converter and addressable by respectiveones of the down-converted microwave signals.

In accordance with another broad aspect, there is provided a quantumcomputing system comprising a quantum processor disposed inside acryogenics apparatus, the quantum processor comprising a plurality ofsolid-state quantum circuit elements configured for outputting microwavesignals, and at least one microwave-to-optical converter coupled to theplurality of solid-state quantum circuit elements and configured forup-converting the microwave signals to optical frequencies. The quantumcomputing system also comprises at least one optical fiber coupled tothe quantum processor for carrying the up-converted microwave signals,and a quantum control system configured for receipt of the up-convertedmicrowave signals.

In accordance with yet another broad aspect, a method for operating aquantum computing system. The method comprises generating microwavesignals up-converted to optical frequencies, carrying the up-convertedmicrowave signals to a quantum processor using at least one opticalfiber, down-converting, at the quantum processor, the up-convertedmicrowave signals, and addressing solid-state quantum circuit elementsin the quantum processor with the down-converted microwave signals.

In accordance with yet another broad aspect, a method for operating aquantum computing system. The method comprises outputting microwavesignals from a plurality of solid-state quantum circuit elements insidea quantum processor, up-converting the microwave signals to opticalfrequencies, carrying the up-converted microwave signals over at leastone optical fiber, and receiving the up-converted microwave signals at aquantum control system.

Features of the systems, devices, and methods described herein may beused in various combinations, in accordance with the embodimentsdescribed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a solid-state quantum computing system in accordance withthe prior art;

FIG. 2 shows an example embodiment of a system which uses aphotonics-based system to control or measure an array of solid-statequbits;

FIG. 3 shows an example embodiment of a system which uses multiplexingto control or measure an array of solid-state qubits using a singleoptical fiber link;

FIG. 4 shows an example embodiment of a system which uses modulation ofoptical frequency combs as a multiplexing method to control or measurean array of solid-state qubits using a single optical fiber link;

FIG. 5 shows an example embodiment of a system which uses an opticalarbitrary wave generator to achieve multiplexing to control or measurean array of solid-state qubits using a single optical fiber link;

FIG. 6 shows an example of a system which uses an amplifier to amplifythe input control or readout signal after it has been converted fromoptical frequencies to microwave frequencies.

FIG. 7 shows an example of a system which uses a single coaxial cable tosupply a microwave pump signal to an array of amplifiers.

FIG. 8 shows an example of a quantum processor in which opticalcircuitry and qubits are integrated on the same substrate; and

FIG. 9 shows an example of a multi-chip quantum processor in whichqubits are fabricated on one substrate and optical circuitry isfabricated on a second substrate.

DETAILED DESCRIPTION

The present disclosure is directed to a scalable solid-state quantumcomputing platform where the quantum processor is operated at ultra-lowtemperatures and the need to use a significant number of coaxial cablesis alleviated. The connection between the quantum processor and thequantum control system is achieved using optical fibers, and sendingcontrol or readout pulses to the solid state quantum processor operatedat ultra-low temperatures uses a photonics approach.

FIG. 1 shows an example embodiment of a solid-state quantum computingsystem 100 in accordance with the prior art. The system 100 comprises aquantum processor 110 placed inside a cryogenics apparatus 120 such as adilution fridge, and cooled down to ultra-low temperatures. Control andreadout of the qubits in the quantum processor 110 is performed by aquantum control system 160. The quantum control system 160 may itselfhave separate modules for uplink and downlink. As used herein, anysignal originating from the quantum control system 160 and delivered tothe quantum processor 110, either for qubit control or readout, isreferred to as “uplink” and any signal originating from the quantumprocessor 110 and delivered to the quantum control system 160 isreferred to as “downlink”. In FIG. 1 , an uplink module 140 generatesthe necessary microwave pulses for qubit control and readout. Themicrowave pulses are transmitted to the quantum processor 110 via a setof coaxial lines 170A that are thermally anchored to cold stages 121 ofthe cryogenic apparatus 120 through a set of attenuators 122. Qubitstates are measured through a set of coaxial lines 170B that leave thequantum processor 110 and connect to the downlink module 150 which is atroom temperature and outside the cryogenics apparatus 120. The set ofcoaxial lines 170B are also thermally anchored to the cold stages 121 ofthe cryogenics apparatus 120 through the set of attenuators 122. Thisqubit readout chain may also involve one or more amplifiers 180 and/orcirculators 190 to further improve the signal to noise ratio and protectthe qubits from microwave feedback.

The quantum processor 110 requires at least one control coaxial line perqubit on top of a number of readout coaxial lines. Additional coaxiallines can also be required to control couplers used to implementmulti-qubit gates. These coaxial lines are bulky, expensive, and alsoresult in heat leak from the hotter stages of the cryogenics apparatus120 to the colder areas. Noting that the cooling power of the cryogenicsapparatus 120 decreases with temperature, installing more than a fewhundred coaxial lines is challenging as the heat leak from the coaxiallines may exceed the cooling power of the cryogenics apparatus 120.

Moreover, practical applications of quantum computers typically requirehundreds of thousands, and even millions, of qubits. Therefore, thesolid-state quantum computing system 100 in accordance with the priorart is not scalable for large scale quantum processors. The presentdisclosure addresses these shortcomings by making use of optical fibers,which result in significantly lower heat load compared to coaxialcables. Optical fibers also provide large bandwidth which allows the useof multiplexing to address a large number of qubits with a single fiber.

FIG. 2 illustrates an example quantum computing system 200 in accordancewith the present disclosure. The system 200 comprises a quantumprocessor 210 which is housed inside a cryogenics apparatus 220 such asa dilution fridge. The quantum processor 210 comprises one or moreoptical-to-microwave converters 211 which down-convert optical signalsto microwave signals. The microwave signals are delivered to one or morequantum circuit elements 212 such as solid-state qubits and couplersoperating in a sub-Tera Hertz frequency band. Microwave signalsoriginating from quantum circuit elements 212 are up-converted tooptical frequencies by one or more microwave-to-optical converters 213.The quantum processor 210 is connected to a quantum control system 260through one or more fiber optic links 230. The quantum control system260 may itself have separate subsystems for uplink, such as uplinkmodule 240, and downlink, such as downlink module 250. In someembodiments, the quantum control system 260 comprises one integratedsystem for uplink and downlink. In some other embodiments, the quantumcontrol system 260 comprises physically separate subsystems for uplinkand downlink.

Optical-to-microwave converters 211 may for example consist ofphotodiodes, such as InGaAs photodiodes, or single-photon detectors.Microwave-to-optical converters 213 may for example consist of opticalphase modulators, such as LiNbO₃-based electro-optical phase modulators,or other transducers based on optomechanics, piezo-optomechanics,electrooptics or magneto-optics.

In some embodiments, parts or all of the quantum control system 260and/or its subsystems may reside inside the cryogenics apparatus 220.

Instead of pulse shaping microwave signals and delivering them to aquantum processor through coaxial cables, the quantum control system 260modulates and demodulates optical signals that are delivered to ororiginate from the quantum processor 210 through optical fibers 230. Theoptical signals may have a wavelength in the short-wave infrared band,for which the transmission of optical fibers is maximal.

In some embodiments, one or more multiplexing schemes, such asWavelength-Division Multiplexing (WDM), may be used to control and/ormeasure multiple qubits at the same time. FIG. 3 illustrates an exampleembodiment of the uplink side of a quantum computing system 300implementing a multiplexing scheme. In this example, an uplink module340 comprises an array of optical sources 341 followed by an array ofmodulators 342 before the optical signals are combined using amultiplexer into an optical fiber 330 and multiplexed optical signalsare delivered to a quantum processor 310 located inside a cryogenicsapparatus 320.

FIG. 4 illustrates another example of the uplink side of a quantumcomputing system 400 where an uplink module 440 uses a frequency combsource 441 rather than an array of optical sources. In one exampleimplementation, the frequency comb source 441 may comprise a mode-lockedlaser which is self-referenced. In another example implementation, thefrequency comb source 441 may rely on strong electro-optic phasemodulation of a continuous laser to generate the frequency comb. In yetanother example implementation, the frequency comb source 441 maycomprise a light source such as a continuous laser connected to anonlinear (Kerr) micro-resonator which creates a frequency comb throughnonlinear mixing. In all of these examples, multiplexed optical signalsare used to address multiple qubits, whereby signals of differentfrequencies are used to address different qubits.

In one implementation, the frequency comb generated by the source 441may pass through a demultiplexer 442 which separates the comb lines androutes them to individual modulators 443 (e.g. a Mach-Zehnder modulator)for pulse shaping. Each frequency line in the frequency comb may beintended for preforming an operation on an individual quantum circuitelement (e.g. a qubit or a coupler) located in a quantum processor 410.The individual modulators 443 are used to provide pulse shaping onindividual frequencies according to the particular operation intended tobe performed on a respective quantum circuit element in the quantumprocessor 410. The channels are then recombined by a multiplexer 444before transmission over an optical fiber 430 to the quantum processor410 located inside a cryogenics apparatus 420.

FIG. 5 illustrates another example embodiment of the uplink side of aquantum computing system 500 in which an uplink module 540 comprises afrequency comb source 541 and an optical arbitrary wave generator (OAWG)542, such as a line-by-line pulse shaper, which translates quantumoperations to waveforms. The multiplexed optical signal output by thegenerator 542 is delivered via an optical fiber 530 to a quantumprocessor 510 located inside a cryogenics apparatus 520.

FIG. 6 illustrates another example embodiment of the uplink side of aquantum computing system 600 which allows the use of a lower opticalpower in an uplink module 640 and optical fibers 630 to avoid excessiveheating in a quantum processor 610 located inside a cryogenics apparatus620. Since the passive heat load (i.e. due to heat propagating along thefiber) of optical fibers is negligible, what may limit the scalabilityof optically controlled quantum computers is the active heat load, i.e.heat due to the dissipation of the optical power at the exit of theoptical fiber. The quantum processor 610 comprises a microwave amplifier614 between an optical-to-microwave converter 611 and quantum circuitelements 612. In some embodiments, the microwave amplifier 614 can be aquantum-limited parametric amplifier, such as a Josephson ParametricAmplifier (JPA) or a Travelling-Wave Parametric Amplifier (TWPA), whichare designed not to introduce any additional noise.

FIG. 7 illustrates a system 700 having a plurality of optical fibers 730and a plurality of optical-to-microwave converters 711A, 711B, 711C. Themicrowave signal generated by each optical-to-microwave converter 711A,711B, 711C is amplified by a respective amplifier 714A, 714B, 714Cbefore being directed to quantum circuit elements 712 of a quantumprocessor 710 located inside a cryogenic apparatus 720. To minimize thenumber of coaxial cables in the cryogenic apparatus 720, a singlecoaxial cable 750 can be used to supply a microwave pump signal to theamplifiers 714A, 714B, 714C. In FIG. 7 , an uplink module 740 providesboth control optical signals and the microwave pump signal, but thesesignals could also be provided by different and separate modules.Alternatively, with an all-optical quantum control system, the microwavepump signal could also be generated from an optical signal provided byoptical fibers and down converted to microwave frequencies by anoptical-to-microwave converter.

FIG. 8 depicts a quantum processor 800 according to one embodiment. Thequantum processor 800 comprises at least one substrate 810. Solid-statequantum circuit elements, such as qubits and couplers, are fabricated onthe substrate through a series of nanofabrication techniques such aslithography, deposition, etching, and lift off. In one exampleembodiment, these quantum circuit elements are fabricated on one side ofthe substrate as part of layer 820. The quantum processor 800 alsocomprises microwave circuit elements, such as readout resonators,microwave filters and transmission lines, which may also be fabricatedon the substrate 810 through a series of nanofabrication techniques suchas lithography, deposition, etching, and lift off. The microwave circuitelements may be fabricated on either side of the substrate 810 and/or aspart of layer 820. The quantum processor 800 may also include anotherset of integrated photonic element such as waveguides, ring resonators,and optical-to-microwave converters. These photonics elements may befabricated in layer 830, on the same side of the substrate as thequantum circuit elements, and/or on the opposite side of the substratein layer 840. In the case where there are elements on both sides of thesubstrate, the electrical connection between both sides of the substratemay be achieved using vias 850. The integrated photonics circuitry maybe connected to a single or an array of optical fibers 860A, 860B fordata transmission.

FIG. 9 depicts a quantum processor 900 according to another embodiment.The quantum processor comprises at least one multi-chip module (MCM)970. In this arrangement, quantum circuit elements may be fabricated inlayer 940 on a first substrate 910 while another substrate 920 carriesanother segment of circuitry and elements such as photonic elements inlayer 960. Microwave circuit elements may reside on either substrate910, 920, in layer 940 and/or layer 950. Elements fabricated ondifferent substrates in layer 940 and layer 950 are electricallyconnected through a plurality of bond bumps 930, such as superconductingbond bumps. The multi-chip module 970 can also include additionalsubstrates, for example a third substrate hosting one or a plurality ofamplifiers (not shown). Amplifiers may also be fabricated on substrate920, on either side thereof. It will be understood that other variantsof 3D integration of such multi-chip quantum processors are possible,including but not limited to multi-chip vertical stack, flip-chip, anddie on wafer arrangements.

The above description is meant to be exemplary only, and one skilled inthe art will recognize that changes may be made to the embodimentsdescribed without departing from the scope of the disclosure. Stillother modifications which fall within the scope of the presentdisclosure will be apparent to those skilled in the art, in light of areview of this disclosure.

Various aspects of described herein may be used alone, in combination,or in a variety of arrangements not specifically discussed in theembodiments described in the foregoing and is therefore not limited inits application to the details and arrangement of components set forthin the foregoing description or illustrated in the drawings. Forexample, aspects described in one embodiment may be combined in anymanner with aspects described in other embodiments. The scope of thefollowing claims should not be limited by the embodiments set forth inthe examples, but should be given the broadest reasonable interpretationconsistent with the description as a whole.

1. A quantum computing system comprising: a quantum control systemconfigured for generating microwave signals up-converted to opticalfrequencies; at least one optical fiber coupled to the quantum controlsystem for carrying the up-converted microwave signals; and a quantumprocessor disposed inside a cryogenics apparatus and coupled to the atleast one optical fiber for receipt of the up-converted microwavesignals, the quantum processor comprising: at least oneoptical-to-microwave converter configured for down-converting theup-converted microwave signals; and a plurality of solid-state quantumcircuit elements coupled to the at least one optical-to-microwaveconverter and addressable by respective ones of the down-convertedmicrowave signals.
 2. The quantum computing system of claim 1, whereinthe up-converted microwave signals are multiplexed signals.
 3. Thequantum computing system of claim 1, wherein the quantum processorfurther comprises at least one amplifier coupled between the at leastone optical-to-microwave converter and the plurality of solid-statequantum circuit elements, the at least one amplifier configured foramplifying the down-converted microwave signals for delivery to thesolid-state quantum circuit elements.
 4. The quantum computing system ofclaim 3, wherein the at least one amplifier is a quantum-limitedparametric amplifier.
 5. The quantum computing system of claim 3,further comprising a coaxial cable coupled to the at least one amplifierto supply a microwave pump signal thereto.
 6. The quantum computingsystem of claim 1, wherein the up-converted microwave signals have awavelength in a short-wave infrared band.
 7. The quantum computingsystem of claim 1, wherein the at least one optical fiber comprises aplurality of optical fibers, and wherein the at least oneoptical-to-microwave converter comprises a plurality ofoptical-to-microwave converters.
 8. The quantum computing system ofclaim 7, further comprising a plurality of amplifiers, each one of theamplifiers connected between one of the optical-to-microwave convertersand the plurality of solid-state quantum circuit elements.
 9. A quantumcomputing system comprising: a quantum processor disposed inside acryogenics apparatus, the quantum processor comprising: a plurality ofsolid-state quantum circuit elements configured for outputting microwavesignals; and at least one microwave-to-optical converter coupled to theplurality of solid-state quantum circuit elements and configured forup-converting the microwave signals to optical frequencies; at least oneoptical fiber coupled to the quantum processor for carrying theup-converted microwave signals; and a quantum control system configuredfor receipt of the up-converted microwave signals.
 10. The quantumcomputing system of claim 9, wherein the up-converted microwave signalsare multiplexed signals.
 11. The quantum computing system of claim 9,wherein the up-converted microwave signals have a wavelength in ashort-wave infrared band.
 12. The quantum computing system of claim 9,wherein the at least optical fiber comprises a plurality of opticalfibers, and wherein the at least one converter comprises a plurality ofmicrowave-to-optical converters.
 13. A method for operating a quantumcomputing system, the method comprising: generating microwave signalsup-converted to optical frequencies; carrying the up-converted microwavesignals to a quantum processor using at least one optical fiber;down-converting, at the quantum processor, the up-converted microwavesignals; and addressing solid-state quantum circuit elements in thequantum processor with the down-converted microwave signals.
 14. Themethod of claim 13, further comprising amplifying the down-convertedmicrowave signals prior to addressing the solid-state quantum circuitelements.
 15. The method of claim 13, further comprising multiplexingthe up-converted microwave signals onto the at least one optical fiberfor delivery to the quantum processor.
 16. The method of claim 13,wherein generating the microwave signals comprises generating signalsfrom at least one light source and pulse-shaping the signals using atleast one modulator.
 17. The method of claim 13, wherein addressing thesolid-state quantum circuit elements comprises addressing different onesof the solid-state quantum circuit elements with different frequencies.18. A method for operating a quantum computing system, the methodcomprising: outputting microwave signals from a plurality of solid-statequantum circuit elements inside a quantum processor; up-converting themicrowave signals to optical frequencies; carrying the up-convertedmicrowave signals over at least one optical fiber; and receiving theup-converted microwave signals at a quantum control system.
 19. Themethod of claim 18, wherein the microwave signals are up-convertedinside the quantum processor.
 20. The method of claim 18, furthercomprising multiplexing the up-converted microwave signals onto the atleast one optical fiber for delivery to the quantum control system.